A hydrostatic transmission is commonly used for high-torque stepless speed control of a load having as an input a prime mover such as an internal-combustion engine or an electric motor.
The hydrostatic transmission can comprise a variable-displacement axial-piston pump whose input shaft is connected to the prime mover and whose displacement per revolution can be varied by tilting an inclinable plate, e.g. a swash plate, to vary the excursion of the axial pistons of the cylinder drum of this pump. The output element of the transmission can include a hydrostatic motor, i.e. an axial-piston motor whose cylinder drum is rotatable about an axis inclined to the output shaft and whose axial pistons act upon, for example, a drive flange which can lie perpendicular to the axis of the output shaft.
The pump is hydraulically coupled with the motor so that the latter is driven when the pump is rotated. Such hydrostatic transmissions have the advantage that they are able to provide stepless control of the speed ratio between the input shaft and the output shaft.
Because of the advantages of such hydrostatic transmissions, they are frequently incorporated in vehicle systems, e.g. agricultural tractors and building machinery, between the internal-combustion engine and the driven wheels of such vehicles. In this case, the support or chassis of the vehicle may have a driving shaft (drive shaft) which is connected directly or via a clutch or gearing with the crankshaft of an internal combustion engine, and a driven shaft shich is connected directly or via gearing (e.g. a differential) to the driven wheels of the vehicle.
In many cases, a mechanical transmission is desired between the engine and the driven wheels for stepped-speed control of the wheels, a mechanical transmission of this type being provided with shifting gears, which can connect the drive shaft with the driven shaft with gear ratios in predetermined steps.
In the fabrication of vehicles of this type, it is frequently required to switch over from the assembly of mechanical transmission vehicles to hydrostatic-transmission vehicles and vice versa. This has posed a problem heretofore since the housing structures and mounting means for the two different types of transmissions were so different that the switchover had to begin during assembly of the chassis and could not await assembly of the transmission structure itself. This was a significant problem especially in the production of tractors in which the transmission housing is constituted as a supporting part of the rear of the tractor. It was simply not possible heretofore to assemble the tractor apart from the specific transmission structure, place the latter on supports and mount the transmission in place, selecting between a hydrostatic variable-speed transmission and a mechanical stepped-speed transmission.
When attempts were made to solve this problem by designing the components of the hydrostatic transmission so that they could be accommodated in the housing which was otherwise designed to receive the mechanical transmission, the steps of assembling the hydraulic transmission in this housing were time-consuming and problematical.